1. Field of the Invention
The present invention relates to a method for the manufacture of a steel product wherein the steel is subjected to isothermal martensite formation and precipitation hardening in a martensitic structure subsequent to soft annealing and shaping. The invention also relates to a steel product obtained with such method and to the use of said steel product.
2. State of the Art
In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not necessarily be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
In published international patent application WO93/07303, a method of manufacture of the above mentioned kind has been described wherein the transformation into the martensitic structure is accomplished by air cooling after annealing in the austenitic region or by cold working. Air cooling after annealing normally results in the so-called athermal kinetic mode of martensite transformation. The formation of air cooling induced martensite is suppressed by alloying elements like nickel, titanium and aluminum, which are used for precipitation of hardenable steel. It may be that at relatively high concentrations of such alloying elements the austenite is stabilized such that the martensitic transformation start temperature becomes impracticably low.
It is an object of the invention to offer a method for the manufacture of steel products, steel products so manufactured and the use of said steel products whereby a practical optimum is achieved between ductility, strength, wear and corrosion resistance, homogeneity of martensite distribution and a practical level of the martensite transformation temperature.
One aspect of the present invention is a method for the manufacture of a steel product comprising the steps of subjecting the steel to precipitation hardening in a martensitic structure subsequent to soft annealing. The steel is then shaped into the desired form followed by solution annealing between a temperature of 1200xc2x0 C. and 1050xc2x0 C. and for a time period of from 5 to 30 minutes. From the annealing temperature, the steel is quenched at a rate of at least 5xc2x0 C. per second to a temperature below 500xc2x0 C. The quenched steel being subjected to an isothermal martensitic transformation. Hardening of the steel is then accomplished at a temperature of between 450xc2x0 C. and 550xc2x0 C. for at least 3 minutes to cause precipitation of particles out of solution into the martensitic structure.